Facile synthesis of acylsilanes via aerobic oxidation of gem-disilylalkylcopper compounds [23]

Full text of DOI:10.1021/ja016716r

J. Am. Chem. Soc. 2001, 123, 11109-11110
11109
Scheme 1
Facile Synthesis of Acylsilanes via Aerobic Oxidation
of gem-Disilylalkylcopper Compounds
Atsushi Inoue, Junichi Kondo, Hiroshi Shinokubo, and
Koichiro Oshima*
Department of Material Chemistry
Graduate School of Engineering
Kyoto UniVersity, Kyoto 606-8501, Japan
ReceiVed July 26, 2001
ReVised Manuscript ReceiVed September 27, 2001
Table 1. Optimization of Aerobic Oxidation Conditionsa
Oxidation of organometallic reagents,1 especially organoborane2
and organosilicon3 compounds, is a methodology of great
importance in organic synthesis for the preparation of alcohols.
In general, the oxidation of a carbon-metal σ-bond does not
provide carbonyl compounds (except from oxidation of vinyl-
metals). Knochel pioneered the oxidation of dimetallic compounds
with oxygen to yield ketones (Scheme 1).4 Since then, however,
few reports of the oxidation of organometals to carbonyl
compounds have appeared in the literature.5
Acylsilanes are an important class of compounds that are
frequently utilized as intermediates in organic synthesis.6 Herein
we wish to report an operationally simple oxidation protocol to
prepare various acylsilanes. Oxidation of 1,1-disilylalkylcopper
compounds with atmospheric oxygen furnishes acylsilanes in good
to excellent yields.
yield (%)
(4a/5)
yield (%)
(4a/5)
Cu salt
additive
Cu salt additive
CuCN NH4Cl aq
CuCN‚2LiCl NH4Cl aq
CuCN‚2LiCl NH3 aq
CuCN‚2LiCl Et3N
84/4
71/5
65/5
66/5
72/12
52/18
35/32
0/51
CuI
NH4Cl aq
CuBr NH4Cl aq
CuCl2 NH4Cl aq
CuCN‚2LiCl pyridine
a The reaction mixture was exposed to air. After oxidation for 30
min, the reaction was quenched with concentrated HCl.
gem-Disilylalkylcopper (3a) was easily prepared via trans-
metalation of 1,1-bis(methyldiphenylsilyl)hexyllithium (2), which
was obtained by the treatment of 1,1-disilylethene (1)7 with
butyllithium in THF. After the addition of aqueous ammonium
chloride, the reaction mixture was exposed to air with stirring
for 30 min. During this period, the aqueous layer turned blue,
indicating the presence of copper(II). After purification, hex-
anoylsilane (4a) was isolated in 84% yield (Table 1).
Scheme 2
In the presence of various additives, oxidation by atmospheric
oxygen afforded the desired acylsilane. Among copper salts we
examined, the THF-soluble complex CuCN‚2LiCl proved to be
the best.8 Aqueous NH4Cl was quite effective as an additive. We
confirmed by 13C NMR spectroscopy of the organic phase of the
reaction mixture that gem-disilylalkylcopper (3a) was not hydro-
lyzed by aqueous ammonium chloride.9 The stability of gem-
disilylalkylcopper toward hydrolysis allows us to employ atmo-
spheric air.
Having optimized the oxidation conditions, we directed our
attention toward the synthesis of a variety of acylsilanes. However,
the current method is limited because only organolithiums can
add to 1,1-disilylethene. Accordingly, we investigated an alterna-
tive method to prepare 1,1-disilylalkylmetals. To our delight, the
treatment of (Ph2MeSi)2CCl2 (6)10 with n-butyllithium followed
by a Grignard reagent in the presence of a copper salt efficiently
furnished the requisite organocopper species 3 (Scheme 2).11 This
copper-mediated alkylative metalation of 7 enables incorporation
of a variety of alkyl groups into the gem-disilylalkylcopper
intermediate.12
Oxidation of the resultant organocopper species with air yielded
the respective acylsilanes in good yields (Table 2).13 Methyl-
diphenylsilanol (8) was formed as a byproduct in almost quantita-
tive yield. Several features of this reaction are noteworthy. Various
primary or secondary Grignard reagents can be employed in the
reaction. The use of silylmethylmagnesium successfully afforded
R-silyl acylsilane (4i). Interestingly, the reaction with crotyl-
magnesium chloride yielded 3-pentenoylsilane (4k) without
2-methyl-3-butenoylsilane (entry 10). The oxidation of disilyl-
(1) Kitching, W. In ComprehensiVe Organic Synthesis; Trost, B. M.,
Fleming, I., Eds.; Pergamon Press: New York, 1991; Vol. 7, Chapter 4.2, p
613.
(2) Pelter, A.; Keith, S. In ComprehensiVe Organic Synthesis; Trost, B.
M., Fleming, I., Eds.; Pergamon Press: New York, 1991; Vol. 7, Chapter
4.1, p 593.
(3) (a) Colvin, E. W. In ComprehensiVe Organic Synthesis; Trost, B. M.,
Fleming, I., Eds.; Pergamon Press: New York, 1991; Vol. 1, Chapter 4.3, p
641. (b) Fleming, I. Chemtracts, Org. Chem. 1996, 9, 1. (c) Tamao, K.; Ishida,
N.; Ito, Y.; Kumada, M. Organic Syntheses; Wiley: New York, 1993; Collect.
Vol. VIII, p 315.
(4) (a) Knochel, P.; Xiao, C.; Yeh, M. C. P. Tetrahedron Lett. 1988, 29,
6697. (b) Chen, H. G.; Knochel, P. Tetrahedron Lett. 1988, 29, 6701. (c)
Knochel, P.; Xiao, C.; Yeh, M. C. P. Organometallics 1989, 8, 2831.
(5) (a) Nakamura, M.; Hara, K.; Sakata, G.; Nakamura, E. Org. Lett. 1999,
1, 1505. (b) Nakamura, M.; Hara, K.; Hatakeyama, T.; Nakamura, E. Org.
Lett. 2001, 3, 3137.
(6) (a) Hodgson, D. M.; Comina, P. J.; Drew, M. G. B. J. Chem. Soc.,
Perkin Trans. 1 1997, 2279. (b) Na`jera, C.; Yus, M. Org. Prep. Proc. Int.
1995, 27, 385. (c) Cirillo, P. F.; Panek, J. S. Org. Prep. Proc. Int. 1992, 24,
555. (d) Page, P. C. B.; Klair, S. S.; Rosenthal, S. Chem. Soc. ReV. 1990, 19,
195. (e) Ricci, A.; Degl’Innocenti, A. Synthesis 1989, 647.
(7) (a) Seebach, D.; Bu¨rstinghaus, R.; Gro¨bel, B.-Th.; Kolb, M. Liebigs
Ann. Chem. 1977, 830. (b) Inoue, A.; Kondo, J.; Shinokubo, H.; Oshima, K.
Chem. Lett. 2001, 956.
(10) Yoon, K.; Son, D. Y. J. Organomet. Chem. 1997, 545-546, 185.
(11) Kondo, J.; Inoue, A.; Shinokubo, H.; Oshima, K. Angew. Chem. 2001,
113, 2146; Angew. Chem., Int. Ed. 2001, 40, 2085.
(12) This process is not efficient for PhMe2Si or Me3Si analogues. Lithiation
of 6 (Si ) PhMe2Si or Me3Si) is sluggish.
(13) General procedure: A solution of n-BuLi (0.31 mL, 1.6 M in hexane,
0.5 mmol) was added to a THF solution of 6 (239 mg, 0.5 mmol) at -78 °C.
After stirring for 20 min, i-PrMgBr (0.6 mmol, THF solution) and CuCN‚
2LiCl (0.6 mmol, 1.0 M THF solution) were introduced and the mixture was
stirred for 1 h at 0 °C. Hexane (5 mL) and NH4Cl aqueous (10 mL) were
added, and the mixture was exposed to air with stirring for 0.5 h. Extractive
workup and purification afforded 4f (118 mg, 0.44 mmol) in 88% yield. The
procedure can be easily scaled up. The use of 5.0 mmol of 6 provided 4c in
77% yield.
(8) Direct oxidation of 1,1-disilylalkyllithium without conversion to the
corresponding copper intermediate was ineffective, and afforded a complex
mixture.
(9) We observed no significant change to the 13C spectrum of the reaction
mixture upon the addition of water.
10.1021/ja016716r CCC: $20.00 © 2001 American Chemical Society
Published on Web 10/12/2001

11110 J. Am. Chem. Soc., Vol. 123, No. 44, 2001
Communications to the Editor
Table 2. Preparation of Acylsilanes via Aerobic Oxidation of
1,1-Disilylalkylcopper Compounds
Figure 1. The transition states of TS1 for path A and TS2 for path B.
The structures were optimized at the B3LYP/6-31G* level.
Scheme 3
are conceivable. Path A involves migration of one of the silyl
groups to the internal oxygen with elimination of water (10 f
11). In path B, attack of terminal oxygen on silicon yields the
product through a four-membered-ring transition state (or inter-
mediate), as is often seen in the Peterson reaction. To determine
which route is more plausible, we have carried out ab initio
calculations at the B3LYP/6-31G* level.
+
In path A, the transition state TS1 was optimized with NH4
as a proton source (Figure 1). The activation energy was calculated
to be 2.8 kcal/mol. The Si(5)-C(1)-O(2) angle of 92.6° and the
Si(5)-C(1)-O(2)-O(3) dihedral angle of 167.2° indicate that
positive charge on O(2) is stabilized by (σ-p)π conjugation. In
path B, TS2 has a four-membered-ring structure with 23.6 kcal/
mol of the activation energy at the B3LYP/6-31G* level. These
results provide strong support that the oxidation of 1,1-dialkyl-
silylcopper species in aqueous ammonium chloride proceeds via
the mechanism outlined in path A.17
In conclusion, we have developed a simple and efficient
oxidation of organocopper species with atmospheric air to provide
a wide variety of acylsilanes. In this reaction, a copper salt plays
a dual role to facilitate the migration of an alkyl group from the
metal to the adjacent carbon, and to allow for the formation of
peroxides without hydrolysis of the disilylalkyl group. This
protocol does not require the use of hazardous oxidizing agents,
enabling the preparation of various acylsilanes from the corre-
sponding Grignard reagents with only atmospheric air as the
oxidant.
a During silica gel column purification of 4i, the trimethylsilyl group
was cleaved, and only 4h was isolated. b The product was partially
isomerized to 2-butenoylsilane during silica gel column purification.
c NMR yield with dibenzyl ether as an internal standard. d Air was
introduced after an addition of pyridine (1.5 mmol). e Oxidation required
2 h for completion.
phenylmethylcopper under the standard conditions caused con-
siderable hydrolysis of the benzylic copper (entry 11).14 In this
case, however, oxidation in the presence of pyridine yielded
benzoylsilane (4l) in 68% yield. Employing Ph2MeSiLi yielded
disilyl ketone (4m)15 in 40% yield (entry 12).16
Our proposed mechanism for this reaction is outlined in Scheme
3. The reaction of the copper species with O2 in the presence of
NH4Cl furnishes peroxide 9. After formation of 9, two pathways
Acknowledgment. We thank Dr. Hidehiro Sakurai (Osaka University)
for helpful discussions and Mr. Hideki Yorimitsu for help with theoretical
calculations. This work was supported by a Grant-in-Aid for Scientific
Research (No. 12305058) from the Ministry of Education, Culture, Sports,
Science, and Technology, Japan. A.I. acknowledges the JSPS Research
Fellowship for Young Scientists.
(14) Oxidation in the presence of aqueous NH4Cl provided benzoylsilane
in 55% yield and disilylphenylmethane in 16% yield.
Supporting Information Available: General procedures, spectral data
for compounds, and the Cartesian coordinates for TS1 and TS2 optimized
at the B3LYP/6-31G* level (PDF). This material is available free of
charge via the Internet at http://pubs.acs.org.
(15) (a) Brook, A. G.; Jones, P. F.; Peddle, G. J. D. Can. J. Chem. 1968,
46, 2119. (b) Narasaka, K.; Saito, N.; Hayashi, Y.; Ichida, H. Chem. Lett.
1990, 1411. (c) Bestmann, H. J.; Haas, W.; Witzgall, K.; Ricci, A.; Lazzari,
D.; Degl’Innocenti, A.; Seconi, G.; Dembech, P. Liebigs Ann. 1995, 415. (d)
Sakurai, H.; Yamane, M.; Iwata, M.; Saito, N.; Narasaka, K. Chem. Lett. 1996,
841. (e) Ricci, A.; Fiorenza, M.; Degl’Innocenti, A.; Seconi, G.; Dembech,
P.; Witzgall, K.; Bestmann, H. J. Angew. Chem. 1985, 97, 1068; Angew. Chem.,
Int. Ed. Engl. 1985, 24, 1068.
JA016716R
(17) Detailed discussion on the theoretical calculations will be reported in
due course.
(16) Disilyl ketones are reported to be unstable toward oxygen.15d,e